This invention relates to wide band gap emitter transistors and more particularly, to transistors having a superlattice base region providing an effective narrow gap base.
It is well known that transistors having a wide band gap emitter have important advantages in bipolar transistor technology. The wide band gap emitter in an npn transistor provides high emitter injection efficiency by blocking the movement of holes from the base to the emitter for the same electron injection from the emitter to the base. Consequently, it allows relatively lighter emitter doping and heavier base doping which results in reduced emitter capacitance and lower base resistance. At present, the emitter efficiency of homojunction bipolar transistors having equal energy band gaps in the emitter, base and collector, is controlled by the doping levels in the emitter and base. In the prior art devices, the emitter is heavily doped to increase the injection efficiency. However, the amount of doping and hence the efficiency is limited since too heavy a doping reduces the effective band gap of the emitter and permits holes to easily move from the base to the emitter. Also in the prior art devices, the base resistance is reduced by heavily doping the base. However, this increases the inverse injection efficiency. The reduction in base resistance thus results in an increase in base currents.
Another type of wide band gap emitter in the prior art is shown in U.S. Pat. No. 4,119,994 directed to a heterojunction device formed with GaAs having a wide energy band gap for the emitter layer and using Ge having a narrow band gap for the base and collector layers. Alternatively, one can also use AlGaAs and GaAs as the emitter and the base, respectively. Another approach in the prior art is to fabricate the transistor having the base region formed from a superlattice. As shown in U.S. Pat. No. 4,137,542, the superlattice is formed with alternate layers of different material, such as, GaAs and GaAlAs. Another example of the superlattice is disclosed in Surface Science 113(1982) 479-480 disclosing a periodic sequence of n and p doped GaAs layers with intrinsic layers inbetween.
The use of a superlattice in a wide band gap emitter transistor is disclosed in Japanese Pat. No. 58-114455 which discloses a superlattice base being formed of alternate layers of p+ doped GaAsSb and p- doped GaInAs sandwiched between emitter and collector layers of n doped GaInAs.